The application of powder paint, powder coatings, etc to lengths of continuous moving strip material has been achieved through use of electrostatic spray guns and electrostatic application chambers in which powder, in atomized form, is caused to be attracted to the strip through use of charging electrodes positioned. Electrostatic spray guns are limited by the speed at which the strip may move and the rate at which the powder may be applied. Similarly, the electrostatic application chambers are limited by the rate at which powder can be applied to the strip, thus limiting use of these technologies for coating moving strips of material.
One drawback to electrostatic application chambers is the service requirements, either on account of routine maintenance or because the powder needs to be changed, as may occur when the type or color of the powder is changed. In those events, the coating line has to be stopped for an excessively long period, and the electrostatic coating apparatus essentially taken apart. This greatly limits the utility of the powder application system, and also increases the cost of the resulting coated product.
The electrostatic application of powder paint can be advantageously utilized with different non-metals and metals, such as steel and aluminum, and with strips of different widths. The electrostatic powder application system can be fit into a relatively small footprint, thus eliminating the cost and space of accumulator towers and their sophisticated control assemblies. However, it still is preferred that the tail end of one strip be secured to the lead end of the next to be coated strip, in order to maximize productivity of the coating system. Typically, a stitch is used to connect the tail end to the lead end, but the stitch may extend from one or both strips by such an extent that it may damage the electrostatic coater when it passes through the coating apparatus.
Commercially coated strip product must have a uniform coating thickness and a uniform appearance. Rotating auger brushes have in the past been used to move the powder paint from a hopper to a receptacle from which it is drawn and atomized by other rotating brushes. The powder in the receptacle must have a uniform depth, in order to maintain a uniform head assuring uniform removal. Uniform removal is important to uniform deposition, and thus coating thickness or weight and appearance. We have found that auger brushes do not achieve uniform depths of powder in the receptacle, and instead provide more powder proximate the hopper and less powder at the opposite end. Merely increasing the speed of rotation of the auger brush does not solve this problem, and may instead create a different problem due to the sag in the brush which may occur due to its length. Because of brush sag, powder may actually be thrown from the receptacle when the speed of rotation is increased.
Typical continuous web powder coating systems utilize a shoe which cooperates with the rotating feeder and atomizing brushes to guide the powder before its is launched into the coating zone. The shoes in the past have been formed from a plurality of individual shoe segments, which were held together by compressive forces. Such a shoe was relatively lightweight, but the compressive forces were generally insufficient to overcome sag of the shoe due to its length and permitted small gaps to be created at abutting sections. The strips to be coated can be up to 108 inches wide, and the shoe must be at least that length. Efforts to shim the shoe segments and otherwise overcome the effect of sag and the formation of gaps were generally unsuccessful.
Moreover, because of the tight fit of the shoe to the feeder and atomizing brushes, the shoe made cleaning those brushes and the powder application chamber difficult. The brushes and chamber typically are cleaned with pressurized air, a task made difficult because of the presence and location of the shoe.
As noted, the strip material can have a width of up to 108 inches. The brushes, shoe, and other components must therefore have at least a corresponding length. We have found that the atomizing brush, which rotates at a speed sufficiently high to atomize the powder and expel it centrifigally into the electrostatic coating zone, tends to sag at such long lengths. Moreover, when supported by radial bearings, as has typically been done, the brush tended to vibrate excessively due to its natural frequency of vibration. With radial bearings, this was a speed below the operating speed of the brush. The vibrations tended to damage the equipment, and to throw powder from the atomizer in an uncontrolled way.
The typical powder atomizing system also utilizes a “wing” in cooperation with the atomizing brush to direct the atomized powder into the coating zone. Once set, the wing was fixed in position, regardless of whether the orientation was optimum for the powder being applied or the strip being coated.
Powder application systems can be used to electrostatically apply powder to both surfaces of the strip. Sometimes only one surface is to be coated, however. Although the coaters can be arranged in any orientation, a typical orientation is for the strip to move horizontally. In that event, there is a coater for the upper surface and a coater for the lower surface. The electrostatic coating zone is typically a box-like rectangular assembly. Powder tends to accumulate at corners and on flat surfaces. Once sufficient powder has accumulated, then gravity causes the accumulated clump to fall onto the below moving strip. In that event, a portion of the strip has a non-uniform surface, and is not commercially saleable.
The coating thickness is a function of the speed of the strip and the rate at which powder is atomized. Typical coating systems in the past had a single coater, which applied powder to one surface on one pass and to the other on another pass. This was a slow process. Additionally, because the atomizing rate was essentially fixed, then the strip speed was used to regulate coating thickness.